FIG. 1A is a macro level diagram of a basic current feedback amplifier (CFA) 100. FIG. 1B, which is a high level circuit diagram, will be used to describe the basic operation of the current feedback amplifier (CFA) 100, which can be modeled as two ideal voltage buffers 106 and 108, a complementary pair of current mirrors 110 and 112 and feedback and gain resistors RF and RG. When a voltage is applied to the non-inverting input 102, it is immediately buffered t standard non-inverting op-amp configuration, this voltage causes a change in the current flowing through the feedback and gain resistors RF and RG. The input voltage buffer 106 must supply this current; therefore the current must flow through the current mirrors 110 and 112 and is duplicated, causing a ΔI to be applied to a high-impedance node 114. This current, flowing into the high impedance node 114, causes a change in voltage that is then transferred to the CFA output 116 by the second buffer 108. The key thing to focus on in this case is that the amplifier's operation depends on correctly sensing and mirroring the change in current caused by the initial change in input voltage. This creates the “current feedback” nature of the amplifier.
Referring now to FIG. 2, a symmetrical pair of CFAs, labeled 100a and 100b, are hooked up in a standard differential gain configuration to form a differential amplifier circuit 200. Standard analysis using the concept of half-circuits leads to the ability to analyze the differential circuit in terms of two signal paths, one for differential-mode signals and one for common-mode signals (hereafter referred to as DM and CM, respectively). Any arbitrary input to the amplifier circuit 200 can be expressed as a sum of DM and CM components. Half-circuits 300a and 300b are shown explicitly in FIG. 3.
It is immediately apparent that the DM and CM signal paths will have different voltage gains and different loading effects, and that therefore optimizing the amplifier circuit 200 for one of the paths will inevitably compromise optimal performance on the other path. More generally it may be desirable to send completely different signals on the DM and CM signal paths. It may also be desirable to cancel out part or all of either the CM or DM signals. Therefore, the ability to tune the CM and DM paths independently is desirable.